On Wed, Sep 04, 2013 at 10:45:50AM +0100, Chris Wilson wrote: > Whilst running the shrinker, we need to hold a reference as we unbind > the objects, or else we may end up waiting for and retiring requests, > which in turn may result in this object being freed. > > This is very similar to the eviction code which also has to be very > careful to keep a reference to its objects as it retires and unbinds > them. > > Another similarity, that Ben pointed out, is that as we may call > retire-requests, the unbound_list is outside of our control. We must > only process a single element of that list at a time, that is we can not > rely on the "safe" next pointer being valid after a call to > i915_vma_unbind(). > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 > IP: [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915] > PGD 758d3067 PUD ac0d6067 PMD 0 > Oops: 0000 [#1] SMP > Modules linked in: dm_mod snd_hda_codec_realtek iTCO_wdt iTCO_vendor_support pcspkr snd_hda_intel i2c_i801 snd_hda_codec snd_hwdep snd_pcm snd_page_alloc snd_timer snd lpc_ich mfd_core soundcore battery ac option usb_wwan usbserial uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core videodev i915 video button drm_kms_helper drm acpi_cpufreq mperf freq_table > CPU: 1 PID: 16835 Comm: fbo-maxsize Not tainted 3.11.0-rc7_nightlytop_8fdad4_20130902_+ #7977 > task: ffff8800712106d0 ti: ffff880028e4a000 task.ti: ffff880028e4a000 > RIP: 0010:[<ffffffffa0082892>] [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915] > RSP: 0018:ffff880028e4b9e8 EFLAGS: 00010246 > RAX: 0000000000000000 RBX: ffff880145734000 RCX: ffff880145735328 > RDX: ffff8801457353fc RSI: 0000000000000000 RDI: ffff88007597cc00 > RBP: ffff88007597cc00 R08: 0000000000000001 R09: ffff88014f257f00 > R10: ffffea0001d65f00 R11: 0000000000bba60b R12: ffff880149e5b000 > R13: ffff880145734001 R14: ffff88007597ccc8 R15: ffff88007597cc00 > FS: 00007ff5bc919740(0000) GS:ffff88014f240000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 0000000000000008 CR3: 0000000028f4c000 CR4: 00000000001407e0 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > Stack: > 0000000000000000 ffff88007597cc00 ffff8801440d6840 0000000000000000 > ffff880145734000 ffffffffa007c854 0000000000000010 ffff88007597c900 > 0000000000018000 00000000004a1201 ffff88007597cc60 ffffffffa007d183 > Call Trace: > [<ffffffffa007c854>] ? i915_vma_unbind+0xe2/0x1d1 [i915] > [<ffffffffa007d183>] ? __i915_gem_shrink+0xf1/0x162 [i915] > [<ffffffffa007d2ee>] ? i915_gem_object_get_pages_gtt+0xfa/0x303 [i915] > [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915] > [<ffffffffa007cbda>] ? i915_gem_object_pin+0x238/0x5ce [i915] > [<ffffffff812cba5f>] ? __sg_page_iter_next+0x2b/0x58 > [<ffffffffa0082056>] ? gen6_ppgtt_insert_entries+0xf2/0x114 [i915] > [<ffffffffa007fe4b>] ? i915_gem_execbuffer_reserve_vma.isra.13+0x79/0x18d [i915] > [<ffffffffa008017c>] ? i915_gem_execbuffer_reserve+0x21d/0x347 [i915] > [<ffffffffa0080bfb>] ? i915_gem_do_execbuffer.isra.17+0x4f3/0xe61 [i915] > [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915] > [<ffffffffa007e405>] ? i915_gem_pwrite_ioctl+0x743/0x7a5 [i915] > [<ffffffffa0081a46>] ? i915_gem_execbuffer2+0x15e/0x1e4 [i915] > [<ffffffffa000e20d>] ? drm_ioctl+0x2a5/0x3c4 [drm] > [<ffffffffa00818e8>] ? i915_gem_execbuffer+0x37f/0x37f [i915] > [<ffffffff816f64c0>] ? __do_page_fault+0x3ab/0x449 > [<ffffffff810be3da>] ? do_mmap_pgoff+0x2b2/0x341 > [<ffffffff810e49be>] ? vfs_ioctl+0x1e/0x31 > [<ffffffff810e5194>] ? do_vfs_ioctl+0x3ad/0x3ef > [<ffffffff810e5224>] ? SyS_ioctl+0x4e/0x7e > [<ffffffff816f88d2>] ? system_call_fastpath+0x16/0x1b > Code: 52 0c a0 48 c7 c6 22 30 0d a0 31 c0 e8 ef 00 f9 ff bf c6 a7 00 00 e8 90 5d 24 e1 f6 85 13 01 00 00 10 75 44 48 8b 85 18 01 00 00 <8b> 50 08 48 8b 30 49 8b 84 24 88 02 00 00 48 89 c7 48 81 c7 98 > RIP [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915] > RSP <ffff880028e4b9e8> > CR2: 0000000000000008 > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68171 > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > --- > drivers/gpu/drm/i915/i915_gem.c | 31 +++++++++++++++++++++++++------ > 1 file changed, 25 insertions(+), 6 deletions(-) > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c > index fdeecae..c31e0b2 100644 > --- a/drivers/gpu/drm/i915/i915_gem.c > +++ b/drivers/gpu/drm/i915/i915_gem.c > @@ -1695,6 +1695,7 @@ static long > __i915_gem_shrink(struct drm_i915_private *dev_priv, long target, > bool purgeable_only) > { > + struct list_head still_bound_list; > struct drm_i915_gem_object *obj, *next; > long count = 0; > > @@ -1709,23 +1710,41 @@ __i915_gem_shrink(struct drm_i915_private *dev_priv, long target, > } > } > > - list_for_each_entry_safe(obj, next, &dev_priv->mm.bound_list, > - global_list) { > + /* As we may completely rewrite the bound list whilst unbinding > + * (due to retiring requests) we have to strictly process only > + * one element of the list at the time, and recheck the list > + * on every iteration. > + */ > + INIT_LIST_HEAD(&still_bound_list); > + while (count < target && !list_empty(&dev_priv->mm.bound_list)) { > struct i915_vma *vma, *v; > > + obj = list_first_entry(&dev_priv->mm.bound_list, > + typeof(*obj), global_list); > + list_move_tail(&obj->global_list, &still_bound_list); > + > if (!i915_gem_object_is_purgeable(obj) && purgeable_only) > continue; > > + /* Hold a reference whilst we unbind this object, as we may > + * end up waiting for and retiring requests, which may result > + * in this object being freed. > + * > + * Note that the shrinker and eviction is special as they operate > + * on the inactive lists which reference limbo objects. > + */ This confused me a bit (imo makes only sense with an s/inactive/active/). And I think this needs a some clarification to make it clear why we don't need this same trickery for the unbound list. What about this instead? /* * Hold a reference whilst we unbind this object, as we may * end up waiting for and retiring requests. This might * release the final reference (held by the active list) * and result in the object being freed from under us. * in this object being freed. * * Note that only shrinking the bound list is special * since only active (and hence bound objects) can contain * such limbo objects. */ > + drm_gem_object_reference(&obj->base); I also think we need a comment here explaining why it is safe to grab a reference despite that the bound (or unbound list fwiw) doesn't hold a reference itself. Ok if I add /* * Note: Even though the bound list doesn't hold a * reference to the object we can savely grab one here: * The final object unreferencing and the bound_list are * both protected by the dev->struct_mutex and so we won't * ever be able to observe an object on the bound_list * with a reference count equals 0. */ when applying? Cheers, Daniel > + > list_for_each_entry_safe(vma, v, &obj->vma_list, vma_link) > if (i915_vma_unbind(vma)) > break; > > - if (!i915_gem_object_put_pages(obj)) { > + if (i915_gem_object_put_pages(obj) == 0) > count += obj->base.size >> PAGE_SHIFT; > - if (count >= target) > - return count; > - } > + > + drm_gem_object_unreference(&obj->base); > } > + list_splice(&still_bound_list, &dev_priv->mm.bound_list); > > return count; > } > -- > 1.8.4.rc3 > > _______________________________________________ > Intel-gfx mailing list > Intel-gfx@xxxxxxxxxxxxxxxxxxxxx > http://lists.freedesktop.org/mailman/listinfo/intel-gfx -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html